Soil fractions

Natural soils, despite their lack of homogeneity, can be assorted in fractions or subfractions by reference to their particle size. The definition of particle size in this case refers to the maximum size of the particle that is incorporated in the soil. This type of assortment is quite useful to engineers, since it is directly connected to the mechanical behaviour of the soil material. The basic fractions of soils are boulders, cobbles, gravels, sand, silt and clay. 

The representative size of the particles for the above groups and subgroups differs slightly from one specification to another. The representative size of the particles for the above fractions and subfractions in accordance with CEN EN ISO 14688-1 (2013), AASHTO M 146 (2012) and ASTM D 2487 (2011) standards are shown in Table 1.1. 

Boulders and cobbles, gravels and sands are granular soils. Their particles do not have any or almost any cohesion. They are easily recognizable and they are distinguished for their high permeability and good stability under the influence of axial load. The term gravel is 

Silt is soil consisting of very fine particles, which, in contrast with the above groups, have some cohesion. It is rather difficult to visually recognise unless it is dried, broken and sieved using a 0.075 mm sieve (No. 200) or a 0.063 mm sieve. Thus, in this case, silt appears to be in the form of a powder. The silt particles range from 0.002 to 0.063 mm (0.075 mm), that is, larger than clay but smaller than sand particles. The shape of silt particles is mainly spherical. 

Clay is the finest soil material with a particle size less than 0.002 mm. When dispersed in water, it gives a colloid in which the particles are in suspension for a very long time. In contrast to silts and sands, the shape of clay particles is flattened and elongated. Because of the size and nature of particles, a particular mass of clay has the largest specific surface of any other equivalent soil mass. Moreover, the surface of particles is more chemically active as well as unstable than any other soil material. Characteristically, it is reported that in 1 g of clay, there are approximately 90 billion particles, whereas in 1 g of silt and coarse sand (0.5-1.0 mm), there are 5.5 million particles and 700 particles, respectively (Millar et al. 1962). 

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The finer soil fraction contains adsorbed organics, small metallic particles, and bound ionic metals. This fraction may be treated further to remove the contaminants, or it may be incinerated or landfilled. The "clean" coarse fraction may contain some residual metallic fragments. With metal contamination, both the fine and coarse soil fractions may be leached with an acid solution to remove the metals. 

Amino Acids. Early observations on the liberation of amino acids by plant roots were reviewed by Loehwing (94), Rademacher (121), and Borner (12). Free amino acids have been isolated from soil fractions (119), and the excretion of a variety of ninhydrin-positive compounds by plant roots has been demonstrated under controlled conditions by Katznelson et al. (18), Rovira (121), and Pearson and Parkinson (115). 

Drzyzga O, D Bruns-Nagel, T Gorontzy, K-H Blotevogel, E von Low (1998) Incorporation of " C-labeled 2,4,6-trinitrotoluene metabolites into different soil fractions after anaerobic and anaerobic-aerobic treatment of soil/molasses mixtures. Environ Sci Technol 32 3529-3535. 

After the separation stage, the coarse soil fraction is rinsed with clean water to remove residual contaminants and any fine soil particles that may adhere to the coarse particles. Soil washing is not usually a stand-alone technology. Typically, both the fine soil fraction (silts and clays) recovered... 

The flux of a metal in and out of a given soil fraction, during a certain time period of incubation, is calculated as follows (Han, 1998) ... 

The kinetics of transformation of Mn and Fe among soil fractions is related to the redox potential (Eh changes) as well as the content of reductants or oxidants in the soil. Generally, a drop in Eh is observed within a few days of the waterlogging of a soil. In a coastal saline silty-clay soil, Eh was reduced to 210 mV after four days of submergence (Bandyopadhyay and Bandyopadhyay, 1984). The decrease in Eh mobilized Mn, and the maximal soluble Mn concentration was found after 14 days of submergence in a sandy-loam soil (Sadana and Takkar, 1988). 

The initial transformations of Mn among soil fractions during incubation in both moisture regimes were concomitant with changes in pH and Eh. After one hour of saturation incubation, the overall redox potential (pe + pH) was 12.0 and 13.6 in the sandy and loessial soils, respectively (Fig. 6.23). 

In both soils (pe + pH) decreased to a value of about four after 7-9 days of the saturated-paste incubation, and remained stable thereafter. Most of the change resulted from pe changes Eh decreased from 290 mV and 332 mV after one hour of incubation to -200 mV and -190 mV after 7-9 days of incubation in the sandy and loessial soil, respectively, and changed very little thereafter. During the same period, pH in the sandy soil was slightly increased from 7.2 to 7.5-7.7, and in the loessial soil it was decreased slightly from 8.0 to 7.0-7.4. With the decrease of Eh and (pe + pH) of the soils as a result of incubation in the saturated paste condition, Mn was chemically reduced and transformed between soil fractions. The changes in the (pe + pH) of the two soils in the field capacity condition are not available. 

Changes in the percentages of Co in the various soil fractions in the two selected soils during one year of incubation are presented in Fig. 6.27. The details of the rapid initial changes in Co content in its three major active fractions (CARB, ERO, and RO fractions) are presented in Fig. 6.28. 

Shuman L.M. Effects of tillage on the distribution of manganese, copper, iron, and zinc in soil fractions. Soil Sci Soc Am J 1985b 49 1117-1122. 

Shuman L.M., Wang J. Effect of rice variety on zinc, cadmium, iron and manganese content in rhizosphere and non-rhizosphere soils fractions. Commun. Soil Sci Plant Anal 1997 28 23-36. 

Frost RR, Griffin RA (1977) Effect of pH on adsorption of As and selenium from land fill leachate by clay minerals. Soil Sci Soc Am J 41 53—57 Goh K-H, Lym TT (2005) Arsenic fractionation in a fine soil fraction and influence of various anions on its mobility in the sub surface environment. Appl Geochem 20 229-239... 

Bayard, R., Bama, L., Mahjoub, B., Gourdon, R. (1998) Investigation of naphthalene sorption in soils and soil fractions using batch and column assays. Environ. Toxicol. Chem. 17, 2383-2390. 

For agriculture and environment related projects, the <2-mm soil fraction is routinely analyzed and used as a standard, whereas for mineral exploration the <63-pm fraction is used. The latter better reflects bio-accessibility and both fractions are analyzed using different chemical digestions. 

In the Mixed Forest ecosystems a soil fraction less than 1 pm contains most of the elements previously confined in the forest litter and gradually involved in the biogeochemical cycle. In this fraction Cu and Mo forms account for 60-70% of the total soil content. The metals, poorly absorbable by plants, for example, Cr and V, occur in finely dispersed soil fraction in smaller amounts, about 20-30%. 

Step 1 simulates the readily available soil fraction, steps 2-4 indicate potentially available soil fractions, and step 5 yields the unextracted residue and completes the mass balance. Note that the solvent used becomes inaeasingly nonpolar during the extraction sequence. Summary data for the she studied compounds are presented in Fig. 8.48. 

When water flows over a contaminated land surface, pollutants released from higher elevations are transported, as dissolved solute or adsorbed on suspended particles, and accumulate at lower elevations. This behavior is reflected in the spatial variability of contaminant concentration, which affects contaminant redistribution with depth following leaching. If a sorbed contaminant is not of uniform concentration across all soil-size ranges but is higher in the fine sediment fraction, the deposition of this soil fraction controls contaminant redistribution in the subsurface. 

As a regional observation, arsenic is concentrated in the >1 mm soil fraction, and defines broad-scale anomalies in areas of shallow bedrock or residual soil. It has probably been adsorbed onto amorphous secondary Fe oxides, some of which have formed pisoliths. By contrast, Au is generally concentrated in the claysized fraction of transported soils. 

This study documents the chemistry and mineralogy of soil fractions adjacent to the Endeavor Mine area to evaluate how best to use soils in regional exploration in western NSW. 

Soil samples were wet sieved into (a) 2-4 mm, (b) 1-2 mm, (c) 0.5-1 mm, (d) 250-500 i m, (e) 125-250 am, (f) 63-125 j,m and (g) <63 j.m fractions. A ferruginous/magnetic fraction (m) was also prepared from the 2-4 mm fraction. Soil fractions were crushed, digested with HNO3/HCI/HF/HCIO4 and then analysed by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) for Al, Ca, Cu, Fe, K, Mn, Na, P, S and Zn. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was used to determine Ag, As, Cd, Pb and Sb because of the lower detection limits by this method. The mineralogy of selected samples was determined by qualitative X-ray diffractometry. 

Examination of soil fractions coarser than 250 pm indicates that they contain a substantial amount of magnetic material. The proportion of magnetic to nonmagnetic (lithic) material generally... 

As indicated above, Pb is preferentially concentrated in coarse and magnetic soil fractions, with Zn concentrated in the finer soil fraction. Dunlop et al. (1983) also observed this separation of Pb and Zn in... 

Table 1. Geochemistry of surficial soil fractions, Endeavor Mine area...

Ferruginous, coarse Pb-rich Bengaccah anomaly material would have been deposited over a relatively short interval in a palaeochannel and subsequently covered by later depositional material. However fermginous material (commonly pisoiiths) in the current stream channels reflects material transported and reworked over the period since the Tertiary. Thus the anomalous Pb in surficial coarse and magnetic soil fractions from the Bengaccah and Northern Pods area represents Tertiary to present mechanical dispersion. 

Zinc is elevated in the fine (<63 jm) soil fractions relative to coarse and magnetic soil fractions but high values (>100 ppm) are found in the fine fraction of surficial soils in the Bengaccah area and immediately above the 450 m-deep Northern Pods mineralisation. Fine material from soils 250 m east of the latter are not so enriched. This suggests that the Zn is not particularly mobile, possibly reflecting derivation from a very local source, possibly siderite up dip from the Northern Pods and Bengaccah. 

Although Zn contents are highest in the fine fraction of the soils, the time needed to prepare such a soil fraction makes its use impractical. However, because fine generally makes up 35-50 % of the whole soil in the Endeavor, Hera and Wagga Tank areas, it may be able to be used directly. For example, Zn >150 ppm occurs in the highly anomalous samples above Northern Pods mineralisation relative to <40 ppm in a sample 250 m away. However, one would need to determine details of the soil size distribution in other areas being explored. 

For the four herbicides, the soil fraction in the sample bottle was extracted by adding 50 mL of diethyl ether followed by agitation for 15 minutes on a wrist-action shaker. The diethyl ether was decanted and the high concentration samples were extracted three more times with 50 mL of diethyl ether by hand shaking the capped bottles for 2 to 3 minutes. The low concentration samples were extracted two additional times with 25 mL of diethyl ether. 

Phosphorus occurs in various soil fractions as soil minerals combined with Ca, Fe, Al, which are of low solubility bound to particle surfaces of, e.g. sesquioxides, calcite, to Al on humus surfaces in soil solution in the organic matter, primarily as esters. 

Discussion. The colloidal clay and humus soil fractions are negatively charged and therefore attract and adsorb positive ions (cations) on to exchange sites. These may be the so-called basic cations defined above, or the acidic cations H+ and Al +. These cations are not soluble in water when in the adsorbed state, but can exchange with H+ which is present in the acidic vicinity of the plant root system. They are now in solution and able to be absorbed into the plant. The extent to which the exchange sites are saturated with cations, together with the ratios of the cations to each other, indicates the nutrient supplying power of the soil. 

The BioTrol soil washing system is a patented, water-based volume reduction process used to treat excavated soil. It separates slightly contaminated, coarse, washed soil particles from heavily contaminated fine soil particles. The process operates on the premise that (1) contaminants tend to be concentrated in the fine size fraction of soil (sUt, clay, and soil organic matter) and (2) contaminants associated with the coarse soil fraction (sand and gravel) are primarily surficial. The BioTrol soil washing system can be used to treat soils contaminated with petroleum hydrocarbons, pesticides, polychlorinated biphenyls (PCBs), various industrial chemicals, and metals. 

The process is economically attractive only when fines do not make up a high fraction of the soil and where the washed coarse soil fraction meets cleanup requirements and can be returned... 

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